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Further work on the content of the "Rationale" section.

wileyhy
2024-10-06 02:51:32 -07:00
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SC1014.md

@@ -1,16 +1,24 @@
## Use `if cmd; then ..` to check exit code, or `if [ "$(cmd)" = .. ]` to check output. ## Use `if cmd; then ..` to check exit code, or `if [ "$(cmd)" = .. ]` to check output.
### Problematic code: ### Problematic code:
```sh ```sh
# WRONG
if [ grep -q pattern file ] if [ grep -q pattern file ]
then then
echo "Found a match" echo "Found a match"
fi fi
``` ```
### Correct code: ```sh
# WRONG
if ! [[ logname == $LOGNAME ]]
then
echo "Possible `su` shell"
fi
```
### Correct code:
```sh ```sh
if grep -q pattern file if grep -q pattern file
then then
@@ -18,20 +26,40 @@ then
fi fi
``` ```
```sh
if ! [[ $(logname) == $LOGNAME ]]
then
echo "Possible `su` shell"
fi
```
### Rationale: ### Rationale:
`[ ... ]` as shell syntax is a simple command that tests for whether certain conditions are true or false, such as whether the value assigned to a variable has a non-zero length (`[ -n "${foo}" ]`) or whether a file system object is a directory (`[ -d "${dir}" ]`). `If-then-(elif-then)-else-fi` statements are logical constructs which themselves contain lists of commands which can include simple commands. `[ ... ]` as shell syntax is a simple command that tests for whether certain conditions are true or false, such as whether the value assigned to a variable has a non-zero length (`[ -n "${foo}" ]`) or whether a file system object is a directory (`[ -d "${dir}" ]`). `If-then-(elif-then)-else-fi` statements are logical constructs which themselves contain lists of commands which can include simple commands.
`[` is just regular command, like `whoami` or `grep`, but with a funny name (see `ls -l /bin/[`). It's a shorthand for `test`. `[[` is similar to both `[` and `test`, but `[[` offers some additional unary operators, such as '=~' the regular expression comparison operator. It allows one to use extglobs such as `@(foo|bar)` (a "bashism"), among some other less commonly used features. `[` is just regular command, like `whoami` or `grep`, but with a funny name (see `ls -l /bin/[`). It's a shorthand for `test`. `[[` is similar to both `[` and `test`, but `[[` offers some additional unary operators, such as '=~' the regular expression comparison operator. It allows one to use extglobs such as `@(foo|bar)` (a "bashism"), among some other less commonly used features.
`[[`, `[` and `test` are often used within `if...fi` constructs in the conditional commands position: which is between the 'if' and the 'then.' `[[`, `[` and `test` are often used within `if...fi` constructs in the conditional commands position: which is between the 'if' and the 'then.'
There are certain shell syntaxes which can be wrapped directly around simple commands, in particular: (1) `{ ...;}` compound command groupings, (2) `$( ... )` command substitutions and (3) `<( ... )` and `>( ... )` process substitutions. Some examples include `{ [ -d "${HOME}/file" ]; echo $?;}`, `var=$(logname)` and `readarray -d "" -t files < <( find ~ -iname '*bash*' -print0 2> /dev/null )`, respectively. However, the various testing commands' syntaxes are not to be included among this list. There are certain shell syntaxes which can be wrapped **directly** around simple commands, in particular:
- (1) `{ ...;}`, group commands,
- (2) `$( ... )`, command substitutions,
- (3) `<( ... )` and `>( ... )`, process substitutions,
- (4) `( ... )`, subshells, and
- (5) `$(( ... ))` and `(( ... ))`, arithmetic evaluations.
Some examples include:
- (1) `{ echo {a..z}; echo {0..9};} > ~/f`,
- (2) `[[ $(logname) == $LOGNAME ]]`,
- (3) `readarray -t files < <( find ...)`,
- (4) `(cd /foo || exit 1; tar ...)`, and
- (5) `dd bs=$((2**12)) count=1 if=/dev/zero of=/tmp/zeroed-block`, respectively.
Note how in example (2) `logname` is enclosed **directly** within a command substitution, which is itself enclosed within a `[[` reserved word / conditional expression / compound command.
If you want to check the exit status of a certain command, use that command directly as demonstrated in the correct code, above. If you want to check the exit status of a certain command, use that command directly as demonstrated in the correct code, above.
If you want to check the output of a command, use `"$(..)"` to get its output, and then use `test` or `[`/`[[` to do a string comparison: If you want to check the output of a command, use `"$(..)"` to get its output, and then use `test`/`[` or `[[` to do a string comparison:
```sh ```sh
# Check output of `whoami` against the string `root` # Check output of `whoami` against the string `root`
@@ -41,13 +69,15 @@ then
fi fi
``` ```
For more information, see [this problem in the Bash Pitfall](http://mywiki.wooledge.org/BashPitfalls#if_.5Bgrep_foo_myfile.5D) list, or generally [Tests and Conditionals](http://mywiki.wooledge.org/BashGuide/TestsAndConditionals) in the WoolEdge BashGuide
### Exceptions: ### Exceptions:
None. None.
### Related resources:
### Related Resources:
For more information, see [this problem in the Bash Pitfall](http://mywiki.wooledge.org/BashPitfalls#if_.5Bgrep_foo_myfile.5D) list, or generally [Tests and Conditionals](http://mywiki.wooledge.org/BashGuide/TestsAndConditionals) in the wooledge.org BashGuide
* [How do I use a file grep comparison inside a bash if/else statement?](https://stackoverflow.com/questions/2480584/how-do-i-use-a-file-grep-comparison-inside-a-bash-if-else-statement) * [How do I use a file grep comparison inside a bash if/else statement?](https://stackoverflow.com/questions/2480584/how-do-i-use-a-file-grep-comparison-inside-a-bash-if-else-statement)
* [Bash Pitfalls: `if [grep foo myfile]`](https://mywiki.wooledge.org/BashPitfalls#if_.5Bgrep_foo_myfile.5D) * [Bash Pitfalls: `if [grep foo myfile]`](https://mywiki.wooledge.org/BashPitfalls#if_.5Bgrep_foo_myfile.5D)